Also factor out CGI-related code from http.c, it's getting too big.

Build tests with -Wundef

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"#if FOO" still works with simple -DFOO, but gives more flexibility.
Specifically, if user expressed no preference (FOO is not defined),
we can apply reasonable defaults (this is the legitimate use of ifdef).

In short, from now on, please use

 #if MG_ENABLE_FOO

instead of

 #ifdef MG_ENABLE_FOO

Since we are all used to #ifdef, this change also adds a precommit check
to police this. Specifically, in *.h and *.c files that are Copyright Cesanta,
"ifdef" and "if defined()" are not allowed to be used with macros that contain
ENABLE or DISABLE, unless the like also contains "ifdef-ok".

Hence, if you are sure you want to use ifdef, use this:

 #ifdef MG_ENABLE_FOO /* ifdef-ok */

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esp_mg_net_if was a rat's nest of mongoose net_if and event manager
implementaions and the event loop task for the ESP8266 miot port.
From that, pieces that are related to LWIP support have been pulled out,
separated into net_if and event manager files, and event loop task
has been moved under the miot esp8266 dirrectory.

This is done to facilitate LWIP code reuse. This may not be the end of
it, but it's a start.

Note: custom retransmit logic has been removed for now. Can be
reintroduced later is necessary.

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Currently this allows V7 to be built on MBED; more
work is likely required to port mongoose.

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This is the variant of MG_EV_SSI_CALL with context of the SSI call being
processed in addition to the tag argument. Specifically, call handler
now gets access to the HTTP request and the name of the file being processed.

MG_EV_SSI_CALL is preserved for backward compatibility.

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So they are distinct from CC3200 projects during import

Also update compiler version (5.2.7 -> 5.2.8)

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Main TODOs:
1. the spec requires the impl to resend the reply once and
   to send gratuitous updates

2. advertise our clubby services as DNS-SD sub-services.

---

Tested with:

1. https://play.google.com/store/apps/details?id=com.grokkt.android.bonjour&hl=en
2. https://play.google.com/store/apps/details?id=com.druk.bonjour.browser&hl=en
3. osx `dns-sd`

Works well enough; it doesn't have some glitches due the fact that we
don't retransmit yet and we don't send gratuitous updates.

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Make paths embedded in amalgamated files nice and relative.
This facilitates subsequent clean un- and re-amalgamation.

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